Combustion engines, such as those within a vehicle propulsion system, may be exposed to high temperatures throughout operation. Much of the heat produced during combustion is absorbed by exhaust gas and transported throughout the exhaust system. The exhaust valves of an engine's cylinders may be exposed to the highest temperatures during high load operation. These temperatures can affect the structural integrity of metal valves and valve seats within the engine. Over time, the decreased structural integrity and repetitive impact from valve opening and closing may cause the softening and warping of a valve or valve seat. Continued wear can cause the valve seat to recede into the engine and away from the valve. Valve seat recession can lead to engine overheating, decreased fuel efficiency, increased emissions, and valve degradation.
Valve seat recession is increased in natural gas engines for a number of reasons. Natural gas (NG) has a higher specific heat than gasoline and thus burns at a higher temperature. NG also has a significantly smaller hydrogen-to-carbon ratio than gasoline engines. Thus, NG engines produce less carbon soot to provide thermal insulation and lubrication. At peak output, rich running gasoline engines may operate up to 40% rich to abate valve overheating. In comparison, NG engines operate around 10% at peak conditions. Lean-running NG engines also do not utilize advanced spark timing to lower engine temperatures during high load conditions. For these reasons, traditional gasoline engine methods to prevent valve recession have a diminished effect on NG engines. The inventors have thus developed additional or alternative measures to increase thermal insulation and lubrication for NG engines, reducing valve recession.
In one approach, additives may be injected into NG engines during operation to produce increased lubrication and thermal protection. Further, the inventors have recognized that by injecting an additive in response to an engine valve temperature, valve protection may be increased without unduly diminishing the increased efficiency and decreased emissions of NG engines.
The inventors have also recognized that by coating a metal valve or valve seat in a zinc phosphate, and periodically exposing the valve to a stearate solution, a lubricating and protective coating is formed on the surface of the valve. For example, zinc phosphate coatings applied to ferrous and non-ferrous metals, such as aluminum or steel, can interact with some stearate compounds, such as sodium and potassium stearate, to form zinc stearate within the crystal lattice of the coating. This reaction creates a tightly bonded lubricant and protective coating on the surface of the metal. As such, this coating may act as both a physical and thermal barrier for the valve thus abating valve recession.
In another embodiment, the inventors have identified that by injecting an amount of hydrocarbon additive into the engine prior to combustion, soot production may be increased. However, an advantage of NG engines is reduced emissions resulting in part, by the reduced particulate matter (soot) produced. Thus, by injecting a mass of additive in response to engine or valve temperature, protective soot can be created during selective operating conditions and terminated when increased valve protection is not desired, or when soot production is greater than a threshold.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.